This invention relates to data communications over the Public Switched Telephone Network (PSTN) and more particularly to modem relay devices operating in conjunction with end user modems to permit full duplex exchange of data over voice compressed communications channels.
Telephone calls over a communication channel using voice compression provides for greater bandwidth utilization. Uncompressed pulse code modulated (PCM) voice circuits require the full 64 kb/s bandwidth for voice communications. Utilizing voice compression, the PCM rate can be reduced substantially without appreciably affecting voice quality. Standard voice compression techniques can reduce the PCM rate to 16 kb/s or below. This, of course, results in a better utilization of the 64 kb/s bandwidth which is of particular importance to telephone companies and private enterprises that lease bandwidth from service providers.
Attempts to carry voiceband signals, such as those originating from a fax or modem, over a voice compressed channel results in severe compatibility problems. Obviously, a fax or modem operating at 14.4 kb/s cannot send data over a channel which has been voice compressed to 16 kb/s because the voice compression cannot accurately duplicate the voiceband signal. To overcome this limitation voice compressed channels traditionally employ a secondary system to demodulate and remodulate voiceband signals. For example, when a fax or modem call is initiated a protocol signature is detected and the system stops its operation of compressing the voice and goes into a mode of demodulating and remodulating the voiceband signal. Further, voiceband relay systems as discussed above use a piping approach where the demodulated signal sent out from the near end is reproduced as is at the receiving end.
In half duplex fax calls wherein the hand-shaking protocol is conducted sequentially, echo does not create significant impediments. In full duplex modem calls such as those supported by V.32 ITU and V.34 ITU standards, a ranging operation is conducted as part of their initial protocol exchange. This ranging operation allows the two interconnected modems to calculate the round trip delay of the in-between path and this permits a configuration of the far end echo canceller in order to remove or at least significantly reduce the echo component. Echo components in voice communications are annoying but can be tolerated whereas in data communications they make the system unusable. Line probing is also performed to measure the transfer characteristics of the analog loop.
In a telecommunications system such as might be used by a commercial establishment wherein a PBX is used to connect into the network at the near end and at the far end of the connection a further PBX receives the communication for connection to the end user. If the demodulated signal from the PBX is reproduced as is at the far end PBX, the distance between the near end modem and near end PBX as well as the distance between the near end PBX and far end PBX and the far end PBX and far end modem are not taken into consideration. As a result, the round trip delay is not properly calculated and the modem far end echo canceller cannot operate successfully. In order to make this type of system work effectively the distances between the modem and the PBX must be kept to a minimum to limit the echo within the modem near end echo canceller. This, of course, is not acceptable in most commercial establishments.
The present invention overcomes the aforementioned limitations by incorporating a modem relay in association with the system manager or PBX. The system manager might be, for example, a Newbridge Networks 3600 MainStreet bandwidth manager. The modem relay is configured to exchange parameter information with its associated modem upon detection of the initiation of a modem call. The parameter information includes round trip delay between the modem relay and its associated modem. Other parameter information such as supported data rate, modulation type and encoding algorithm is exchanged. The near end modem relay relays this information to its counterpart modem relay at the receiving end, which, in turn, exchanges parameter information with the designated far end modem. The respective modem relays and modems arrive at compatible parameters and this information is relayed between modem relays such that common transmission characteristics are established. The round trip delay information caused by the two local loops is used to properly configure the echo cancellers. The line probing is also properly performed because the relay modem interacts with its associated modem in a local loop. It is also within the control of the modem relays to not only convey supported parameters but also modify them so as to match the capabilities of the system. For example, if the voice compression uses a bandwidth of 8 kb/s and the modems have agreed on a capability of 14.4 kb/s, the relay can make sure that the information conveyed between modems will not show any data rate above 8 kb/s.
It is an object of the present invention to provide a communications system which permits modem calls over a voice-compressed channel in the Public Switched Telephone Network.
Therefore, in accordance with a first broad aspect of the present invention there is provided in a communications network for transporting data between end point terminals over a communication channel employing voice compression a relay device associated with each end point terminal. Each of the relay devices have means to exchange parameter information with its associated end point terminal and means to relay the parameter information to its counterpart relay device, and to receive parameter information from its counterpart relay device for the purpose of negotiating compatible data transmission conditions.
In accordance with a preferred embodiment the end point terminal is customer premise equipment having voice and data capabilities.
In accordance with a second broad aspect of the present invention, there is provided a method of communicating data from a near end modem to a far end modem over a voice compressed communications channel comprising: providing a modemrelay associated with each modem; configuring each modem relay to exchange parameter information with its associated modem and configuring each modem relay to relay the parameter information to its counterpart modem relay in order to negotiate compatible data transmission conditions.